1. Field of the Invention
This invention relates generally to piezoelectric crystal devices and more particularly, to high frequency, high precision quartz crystals used in applications in which shock, acceleration, and temperature stresses are present.
2. Description of the Prior Art
Piezoelectric crystal resonators are an essential element in frequency control and determination and in other applications where an electrical resonant frequency is used. One of the most common piezoelectrics is quartz which exhibits bulk acoustic waves of the thickness shear type when properly stimulated.
Designers of apparatus employing piezoelectric crystals are often confronted with the problem of devising a suitable mounting method for the crystal. The stability of the resonant frequency of such crystals is adversely affected by shock, acceleration, and even thermal stresses, and the crystal mounting is an avenue through which such stresses may be transmitted directly to the crystal, altering resonant frequency.
Typically, these crystals are fabricated in the form of circular discs; the higher the crystal's resonant frequency, the thinner the crystal disc, and the more susceptible to damage or at least frequency shift.
U.S. Pat. No. 3,694,677 issued to Guttwein, Ballato, and Lukaszek discloses a method of constructing a high frequency resonator while still providing an adequate support structure. The patent shows how to reduce the thickness of a concentric circular portion of the center of a conventional quartz disc while leaving the thickness of the periphery untouched, thus providing an ultrathin inner resonant quartz disc surrounded by an integral, stronger, thicker peripheral support ring. The support ring is relatively robust and may be used for mounting to equipment structure(s). The thickness of the circular center portion is reduced by well known microelectronic techniques, namely photolithography and etching. The process yields an ultrathin resonator with resonant frequencies in the 1000 MHz range which can be easily mounted.
Although the patent discloses a crystal with hitherto unattainably high resonant frequencies, it is still possible for the stresses of acceleration, shock, vibration, and temperature to be transmitted directly to the thin resonant center portion of the disc from the peripheral support ring, thus adversely affecting the crystal's resonant frequency, as mentioned before.
A variety of approaches have been tried to isolate crystal resonators from their mechanical supports to maintain resonant stability. For crystals whose basic configuration is not an ultrathin disc surrounded by an integral support ring, as disclosed in aforementioned U.S. Pat. No. 3,694,677, a variety of approaches have been tried. For example, U.S. Pat. No. 4,454,443, issued to Lukaszek and Ballato, discloses a method of mounting resonators which have uniform thickness and a number of flat sides to which mounting devices may be attached, with the flat sides providing a means of evenly distributing and thus reducing mounting stresses.
French patent No. 79 18553, "Oscillateur haute frequence autothermostate" issued to Besson teaches a method of reducing the stress sensitivity of a quartz resonator by using narrow mounting bridges positioned in accord with crystallographic theory to be discussed below. However the narrow bridges are very fragile and susceptible to breakage.
Other methods used to reduce the stress sensitivity of resonators are discussed in U.S. Pat. No. 4,136,297 issued to Briese. However, all of the methods disclosed teach methods of manufacturing various types of external mounting apparatus such as spring clips, tape, cement, etc., but none teaches a quartz resonator which by its intrinsic shape is acceleration and shock resistant no matter how it is mounted. The present invention teaches such a resonator.
The following publications are pertinent to an understanding of the novel crystal fabrication technique described herein. The significance of each will be described below.
Shockley et al., "Trapped Energy Modes in Quartz Filter Crystals," Jour. Acoustical Soc. of America, vol. 41, No. 4 (Part 2) pp. 981-993, April 1967;
Ballato, "Force-Frequency Compensation Applied to Four-Point Mounting of AT-Cut Resonators," IEEE Trans. Sonics Ultrason., Vol. SU-25, July pp. 223-226, July 1978.
Besson et al "Further Advances on B.V.A. Quartz Resonators," Proc. 34th An. Freq. Control Symposium, U.S. Army Electronics R&D Command, Ft. Monmouth N.J. pp. 175-182 1980;
E. P. Graf et al., "BVA Quartz Crystal Resonator and Oscillator Production," Proc. 37th An. Freq. Control Symposium, U.S. Army Electronics R&D Command, Ft. Monmouth, N.J. pp. 492-500 1983;
Ballato et al "Simplified Expressions for the Stress-Frequency Coefficients of Quartz Plates," IEEE Trans. Sonics Ultrason. Vol. SU-31, pp. 11-18, Jan 1984.
Both experimental and theoretical studies of quartz discs have shown that external forces applied to the periphery of the disc cause vibration frequency changes. Because of the anisotropy of quartz, the same force applied at different azimuth angles may cause different frequency changes. For any given crystal axis orientation a force-frequency may be defined, per Ratajski, referenced in both publications by Ballato supra: EQU K.sub.f (.psi.)=(.DELTA.f/fo) (D 2h/F) (1/N)
where
.psi.=azimuthal angle of applied force measured counterclockwise from crystal X-axis PA1 F=applied compressional force PA1 .DELTA.f=frequency change caused by application of compressional force F PA1 D=resonant disc diameter PA1 2h=thickness of resonant disc PA1 f.sub.o =nominal resonant frequency PA1 N=disc frequency constant (equal to one half the acoustic velocity, or N=f.sub.o .multidot.h)
Apart from the factor N, K.sub.f (.psi.) is the normalized frequency shift divided by the average stress in the cross section. Or, K.sub.f (.psi.) may be regarded as a proportionally factor relating the fractional frequency change, .DELTA.f/fo, to the average stress acting across the crystal diameter, F/(2H.multidot.D). By convention, K.sub.f (.psi.) is considered positive if .DELTA.f is positive when a compressive load is applied, and negative if .DELTA.f is negative.
Quartz resonators may be cut in a variety of ways with respect to the crystal axes. "Singly rotated" and "doubly rotated" refer to cuts oriented with respect to the crystallographic axes. For example, a doubly rotated cut is a sample which is cut at an angle .phi. rotated around the Z-axis of the mother crystal and then cut at an angle .phi. rotated around the already-rotated X axis of the mother crystal. When the angle .phi. is zero, the result is called a singly rotated cut.
The force freqency function, K.sub.f (.psi.), has been either experimentally or theoretically determined for all singly and doubly rotated cuts of current technological interest. For the singly rotated AT cut, for example, the publication "Force-Frequency Compensation Applied to Four-Point Mounting of AT-cut Resonators" above authored by the present inventor, presents both theoretical and experimental data for K.sub.f (.psi.). For doubly rotated cuts, that is, those cuts for which .phi..noteq.0, the publication "Simplified Expressions for the Stress-Frequency Coefficients of Quartz Plates" above, co-authored by the present inventor provides K.sub.f (.psi.) for .phi.=22.4.degree. (SC-cut quartz), and a technique for determining K.sub.f (.psi.) for other cuts currently of lesser interest.
The publication and patent by Besson et al., and the publication by Graf et al., above, disclose a four point stress-compensating mounting for quartz resonators. The resonator is mounted by narrow 0.3 mm bridges. Each bridge encompasses a point at which K.sub.f (.psi.)=0. However, the mounting technique is inherently fragile and will not withstand service stresses or accelerations.